docs: def _add_cloud_pocket(pocket):
      """Add a cloud pocket as /etc/apt/sources.d/cloud-archive.list
  
      Note that this overwrites the existing file if there is one.
  
      This function also converts the simple pocket in to the actual pocket using
      the CLOUD_ARCHIVE_POCKETS mapping.
  
      :param pocket: string representing the pocket to add a deb spec for.
      :raises: SourceConfigError if the cloud pocket doesn't exist or the
          requested release doesn't match the current distro version.
      """
      apt_install(filter_installed_packages(['ubuntu-cloud-keyring']),
                  fatal=True)
      if pocket not in CLOUD_ARCHIVE_POCKETS:
          raise SourceConfigError(
              'Unsupported cloud: source option %s' %
              pocket)
      actual_pocket = CLOUD_ARCHIVE_POCKETS[pocket]
      with open('/etc/apt/sources.list.d/cloud-archive.list', 'w') as apt:
          apt.write(CLOUD_ARCHIVE.format(actual_pocket))